Adds stress tests validating that messages cleared via SequenceReset-GapFill are not subsequently processed by nextQueued(). Related to PR #1133's queue clearing behavior.

Implementation

Two JCStress test classes (332 configurations total):

• ClearBeforeNextQueuedTest: Tests execution ordering between processNextQueued() and dequeueMessagesUpTo(). Validates that messages are either cleared OR processed, never both.

• MultipleSequenceResetTest: Tests multiple concurrent dequeueMessagesUpTo() calls with different sequence numbers to verify queue consistency.

Design notes:

• Synchronized methods respect InMemoryMessageQueue's single-threaded contract ("The map should be accessed from a single thread")
• AtomicInteger tracks processed messages across actor executions
• All sequence numbers extracted to constants (MIN_SEQ_NUM=5, MAX_SEQ_NUM=9, CLEAR_ALL_SEQ_NUM=10)

Test validation

Pre-populates queue with 5 messages (seqnum 5-9). Actor orderings tested:

• Clear-then-process: 0 processed, 0 queued
• Process-then-clear: 1-5 processed, 0 queued
• Partial orderings: N processed, 0 queued where N < 5

All outcomes verify queue is empty post-execution with no messages lost or double-processed.

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.